As conventional materials of contact and terminal parts, such as connectors and switches, there are used plated products wherein a base material of stainless steel, copper, a copper alloy or the like, which is relatively inexpensive and which has excellent corrosion resistance, mechanical characteristics and so forth, is plated with tin, silver, gold or the like in accordance with required characteristics, such as electrical and soldering characteristics.
Tin-plated products obtained by plating a base material of stainless steel, copper, a copper alloy or the like, with tin are inexpensive, but they do not have good corrosion resistance in a high-temperature environment. Gold-plated products obtained by plating such a base material with gold have excellent corrosion resistance and high reliability, but the costs thereof are high. On the other hand, silver-plated products obtained by plating such a base material with silver are inexpensive in comparison with gold-plated products and have excellent corrosion resistance in comparison with tin-plated products.
As a silver-plated product obtained by plating a base material of stainless steel, copper, a copper alloy or the like with silver, there is proposed a metal plate for electrical contacts, wherein a silver plating layer having a thickness of 1 micrometer is formed on a copper plating layer having a thickness of 0.1 to 0.5 μm which is formed on a nickel plating layer having a thickness of 0.1 to 0.3 μm which is formed on the surface of a thin base material plate of stainless steel (see, e.g., Japanese Patent No. 3889718). There is also proposed a silver-coated stainless bar for movable contacts, wherein a surface layer of silver or a silver alloy having a thickness of 0.5 to 2.0 μm is formed on an intermediate layer of at least one of nickel, a nickel alloy, copper and a copper alloy having a thickness of 0.05 to 0.2 μm, the intermediate layer being formed on an activated underlying layer of nickel which has a thickness of 0.01 to 0.1 μm and which is formed on a base material of stainless steel (see, e.g., Japanese Patent No. 4279285). Moreover, there is proposed a silver-coated material for movable contact parts, wherein a surface layer of silver or a silver alloy having a thickness of 0.2 to 1.5 μm is formed on an intermediate layer of copper or a copper alloy having a thickness of 0.01 to 0.2 μm, the intermediate layer being formed on an underlying layer of any one of nickel, a nickel alloy, cobalt or a cobalt alloy which has a thickness of 0.005 to 0.1 μm and which is formed on a metallic substrate of copper, a copper alloy, iron or an iron alloy, the arithmetic average roughness Ra of the metallic substrate being 0.001 to 0.2 μm, and the arithmetic average roughness Ra after forming the intermediate layer being 0.001 to 0.1 μm (see, e.g., Japanese patent Laid-Open No. 2010-146925).
However, when conventional silver-plated products are used in a high-temperature environment, there are some possibility that the adhesion properties of the plating film may be deteriorated and/or the contact resistance of the product may be very high. When the silver-plated products proposed in Japanese Patent Nos. 3889718 and 4279285 are used in a high-temperature environment, there are some possibility that the rise of the contact resistance of the product cannot be sufficiently restrained. On the other hand, when the silver-plated product proposed in Japanese Patent Laid-Open No. 2010-146926 is used in a high-temperature environment, the adhesion properties of the plating film are good, and the rise of the contact resistance of the product can be restrained. However, it is required to adjust the arithmetic average roughness Ra of a pressure roll to be 0.001 to 0.2 μm so that the arithmetic average roughness Ra of a metallic substrate, which is transferred by the pressure roll, is adjusted to be 0.001 to 0.2 μm. It is also required to appropriately choose the current density in plating and the kinds of additives in a plating solution during the formation of the intermediate layer to adjust the arithmetic average roughness Ra to be 0.001 to 0.1 μm after forming the intermediate layer, so that the process is complicated and the costs thereof are increased.
For that reason, the applicant has proposed to produce an inexpensive silver-plated product, which has good adhesion properties of the plating film and which can restrain the rise of the contact resistance of the product even if it is used in a high-temperature environment, by causing the crystalline diameter in a direction perpendicular to {111} plane of the surface layer to be 300 angstroms or more in a silver-plated product wherein a surface layer of Ag is formed on an intermediate layer of Cu which is formed on an underlying layer of Ni formed on the surface of a base material of stainless steel (Japanese Patent Application No. 2010-253045).
However, in a silver-plated product wherein a silver plating film is formed on the surface of a base material of copper or a copper alloy, or on the surface of an underlying layer of copper or a copper alloy formed on a base material, there is a problem in that copper diffuses to form CuO on the surface of the silver plating film to raise the contact resistance thereof if it is used in a high-temperature environment. There is also a problem in that cracks are formed in the silver-plated product to expose the base material if the silver-plated product is worked in a complicated shape or in a shape of small contact and terminal parts, such as connectors and switches.